1. Field of the Invention
The present invention relates to a reactive power compensating apparatus for stabilizing a system voltage of a power system and, more specifically, to a self-commutated reactive power compensating apparatus capable of reducing in switching loss in its steady state.
2. Description of the Related Art
A static var compensator is generally known as an apparatus for stabilizing a system voltage of a power system. In particular, a self-commutated static vat compensator (referred to as a self-commutated SVC hereinafter) is excellent in response characteristic and has the advantage of operating quickly according to variations in power system. The self-commutated SVC varies an output voltage of a self-commutated converter connected to the power system to regulate an amount of reactive power, and then a system voltage. The self-commutated converter generally includes a self-turnoff switching device and a DC (direct current) capacitor for establishing a back power.
FIG. 5 is a view showing a loss of a conventional self-commutated SVC. As shown in this figure, the loss is maximized (point B) at both the maximum points Q.sub.Cmax and Q.sub.Lmax of the reactive power output therefrom, but the loss occurs even when the reactive power is zero (point A). The loss occurring when the reactive power is zero is attributed to a switching operation of a self-commutated converter.
There are many cases where a voltage reactive power controller (VQC) for making a system power constant by opening/closing a power capacitor or a shunt reactor is installed in a substation. It was therefore expected that the reactive power output from the self-commutated SVC in the steady state operation would become substantially zero. Since, however, the conventional self-commutated SVC is always controlled so that the output voltage of the self-commutated converter coincides with the system voltage even in the steady state operation, even if the reactive power output from the self-commutated SVC is substantially zero, the switching operation of the self-commutated converter is repeated, resulting in a switching loss.